Earlier we have shown that nitroso chloramphenicol does not induce aplastic anemia in calves or mice. We have investigated the mechanism by which GSH prevents the inhibition of DNA synthesis by nitroso chloramphenicol in bone marrow cells. GSH reacts nonenzymatically with nitroso chloramphenicol within seconds to form a glutathione conjugate which is converted to the hydroyamine analogue of chloramphenicol. Preincubation of nitroso chloramphenol with GSH completely prevents the inhibition of DNA synthesis induced by the compound indicating the GSH conjugate and the hydroxylamine derivation are not responsible for the inhibition. The ineffectiveness of nitroso chloramphenicol to induce aplastic anemia in animals may be due to conversion of nitroso chloramphenicol to compounds which are not inhibitors of DNA synthesis.